Also because if it were, what you'd have to DDOS is hashpower not nodes-- and lots (hopefully most) hashpower is not directly inbound reachable: it connects out

Also because if it were, what you'd have to DDOS is hashpower not nodes-- and lots (hopefully most) hashpower is not directly inbound reachable: it connects outward, and only gets inbound connections via backup nodes at other locations. This makes it harder to DOS attack since you don't know where it is.